ram speeds that arent supported on my motherboard

Whitizo

Honorable
Sep 20, 2014
56
0
10,630
I need to know fast as I have to order the ram tomorrow.
I have an old pc and im turning it into a decent PC for gaming. Right now I have my PSU and GTX 950. It supports everything but the problem is it only supports up to 1333mhz, I don't wanna buy lower speed ram just to buy a new motherboard in summer time and having slower ram. I was looking to buy 1866mhz but I will be okay with 1600mhz.
So my question is. If I bought a 1600mhz 8gb ram kit for a motherboard that only supports 1333mhz, is it guaranteed to work and just run slower or will there be problems?
I don't want there to be a "chance" it will work. I wanna know if it will definitely work, or if I should just buy the slower ram..